Applicants claim priority of Japanese patent applications Serial No. 2000-350536, filed Nov. 17, 2000, and Serial No. 2000-350537, filed Nov. 17, 2000.
This invention relates to a carburetor, and more particularly to a variable venturi carburetor having a fuel priming cold start device.
In a conventional carburetor a fuel-and-air mixing passage extends usually horizontally through a carburetor body providing a fuel-and-air mixture to the crankcase of a combustion engine. A throttle valve or plate in the passage and near the passage outlet is supported by a shaft carried by the body and extending transversely through the passage, pivots within the passage to control the fuel-and-air mixture flow, which in-part controls the revolutions per minute, rate, of an operating engine. Similarly, a pivoting choke plate is supported within the passage by the body to control the amount of air flow through a venturi with a fixed cross-sectional area disposed in the passage between the throttle and choke plates. A main fuel feed tube communicates transversely into the fuel-and-air mixing passage to emit liquid fuel into the passage for mixing with air. The amount of emitted liquid fuel is dependent upon the amount of vacuum created at the venturi by the operating engine. Typically, for engine idle conditions, a separate fuel nozzle is provided at or near the throttle plate and the main fuel feed tube is reserved for higher speed engine operating conditions.
Unfortunately, for cold engine starts, cold idle and cold acceleration, the operating engine requires a richer or higher ratio of fuel-to-air to start and operate smoothly. Providing the proper additional amounts of fuel for varying air flow amounts for different engine transients (i.e. cranking, idle, and acceleration) is difficult. Often, providing the proper ratio of fuel and air for cold idle conditions will lead to an engine stall during cold acceleration. Furthermore, providing the proper ratio of fuel while maintaining emission performance standards is also difficult.
A variable venturi area carburetor for a combustion engine has an uprighted cup-shaped piston head which forms an integral part of a venturi within a fuel-and-air mixing passage carried by a carburetor body, and a needle that projects rigidly downward from the head into a fuel feed passage that communicates with a fuel chamber at atmospheric pressure. The position of the piston head controls air flow by adjusting the air flow cross-sectional area of the variable venturi, and the needle simultaneously controls fuel flow into the fuel-and-air mixing passage at the venturi via obstruction of the fuel feed passage. The piston head and needle are moved in unison by a flexible diaphragm engaged to and disposed above the head. An atmospheric chamber is defined below the diaphragm and a vacuum chamber is defined generally above the diaphragm. A vacuum pressure passage extends through the bottom of the head communicating between the fuel-and-air mixing passage at the venturi and the vacuum chamber. As vacuum at the venturi increases, the volume of the vacuum chamber decreases and the flexing diaphragm moves the head partially out of the fuel-and-air mixing passage until a balance is reached of forces produced by the vacuum acting on the diaphragm and a resilient compression spring disposed within the vacuum chamber which biases the head into the passage. Retraction of the piston head is opposed by the spring force to increase the magnitude of the vacuum produced by the venturi and thereby creating a rich mixture of fuel-and-air when required. During cold engine starts, cold idling, and cold acceleration, a cold engine priming device senses the temperature of the engine and delivers additional fuel into the fuel-and-air mixing passage from a fuel chamber when the engine temperature is below a pre-set value.
Objects, features, and advantages of this invention include a variable venturi type carburetor which provides an increased quantity of fuel to the fuel-and-air mixing passage when a cranking or running engine is below an optimum running temperature without operator intervention. Another advantage of the present invention is a reliable, robust and relatively inexpensive to manufacture carburetor that causes an engine to start, idle and accelerate smoothly and reliably at cold temperatures without requiring a traditional choke plate or valve.